gpython: Fix gevent activation on Windows

On Windows if geventmp is present gpython startup fails:

    (1.wenv) Z:\home\kirr\src\tools\go\pygo-win\pygolang>gpython
    Traceback (most recent call last):
      File "C:\Program Files\Python310\lib\runpy.py", line 196, in _run_module_as_main
        return _run_code(code, main_globals, None,
      File "C:\Program Files\Python310\lib\runpy.py", line 86, in _run_code
        exec(code, run_globals)
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\Scripts\gpython.exe\__main__.py", line 7, in <module>
      File "Z:\home\kirr\src\tools\go\pygo-win\pygolang\gpython\__init__.py", line 450, in main
        pymain(argv, init)
      File "Z:\home\kirr\src\tools\go\pygo-win\pygolang\gpython\__init__.py", line 223, in pymain
        init()
      File "Z:\home\kirr\src\tools\go\pygo-win\pygolang\gpython\__init__.py", line 428, in init
        _ = monkey.patch_all(thread=patch_thread)      # XXX sys=True ?
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\lib\site-packages\gevent\monkey.py", line 1255, in patch_all
        _notify_patch(events.GeventWillPatchAllEvent(modules_to_patch, kwargs), _warnings)
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\lib\site-packages\gevent\monkey.py", line 190, in _notify_patch
        notify_and_call_entry_points(event)
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\lib\site-packages\gevent\events.py", line 105, in notify_and_call_entry_points
        subscriber(event)
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\lib\site-packages\geventmp\monkey.py", line 160, in _patch_mp
        _patch_module("_mp.3._mp_util", _patch_module=True, _package_prefix='geventmp.')
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\lib\site-packages\geventmp\monkey.py", line 121, in _patch_module
        gevent_module = import_module(_package_prefix + name)
      File "C:\Program Files\Python310\lib\importlib\__init__.py", line 126, in import_module
        return _bootstrap._gcd_import(name[level:], package, level)
      File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
      File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
      File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
      File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
      File "<frozen importlib._bootstrap_external>", line 883, in exec_module
      File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\lib\site-packages\geventmp\_mp\3\_mp_util.py", line 16, in <module>
        from gevent.os import _watch_child
    ImportError: cannot import name '_watch_child' from 'gevent.os' (Z:\home\kirr\src\tools\go\pygo-win\1.wenv\lib\site-packages\gevent\os.py)

That happens because geventmp does not support windows actually.

-> Fix it by requiring geventmp to be present only on non-windows.

Adjust related comment as https://github.com/karellen/geventmp/pull/2 has been merged long ago.

setup.py

@@ -319,10 +319,9 @@ setup(
                         # only runtime part: for dylink_prepare_dso
                         'setuptools_dso >= 2.7',
                         # pyx.build -> setuptools_dso uses multiprocessing
-                        # FIXME geventmp fails on python2, but setuptools_dso
-                        # uses multiprocessing only on Python3, so for now we
-                        # are ok. https://github.com/karellen/geventmp/pull/2
-                        'geventmp;python_version>="3"',
+                        # setuptools_dso uses multiprocessing only on Python3, and only on systems where
+                        # mp.get_start_method()!='fork', while geventmp does not work on windows.
+                        'geventmp ; python_version>="3" and platform_system != "Windows" ',
                        ],
     extras_require   = extras_require,